Batch electric induction heating and melting of an electrical conductive material can be accomplished in a crucible by surrounding the crucible with an induction coil. A batch of an electrically conductively material, such as metal ingots or scrap, is placed in the crucible. One or more induction coils surround the crucible. A suitable power supply provides ac current to the coils, thereby generating a magnetic field around the coils. The field is directed inward so that it magnetically couples with the material in the crucible, which induces eddy current in the material. Basically the magnetically coupled circuit is commonly described as a transformer circuit wherein the one or more induction coils represent the primary winding, and the magnetically coupled material in the crucible represents a shorted secondary winding.
FIG. 1 illustrates in simplified form one example of a circuit comprising a power supply, load impedance matching element (tank capacitor CT), and induction coil LL that can be used in a batch melting process. The power supply 102 comprises ac to dc rectifier 104 and inverter 106. Rectifier 104 rectifies available ac power (AC MAINS) into dc power. Typically after filtering of the dc power, inverter 106, utilizing suitable semiconductor switching components, outputs single-phase ac power. The ac power feeds the load circuit, which comprises the impedance of the induction coil and the impedance of the electromagnetically coupled material in the crucible, as reflected back into the primary load circuit. The value of tank capacitor CT is selected to maximize power transfer to the primarily inductive load circuit. Induction coil LL comprises primary section LP and secondary section LS, which are preferably connected in a counter-wound parallel configuration to establish instantaneous current flow through the coil as indicated by the arrows in FIG. 1.
FIG. 2(a) illustrates the use of the arrangement in FIG. 1 with crucible 110 to batch melt generally solid metal composition 112 (diagrammatically shown as discrete circles) that is placed in the crucible. The state of the batch melting process in FIG. 2(a) is referred to as the “cold state” since generally none of the metal composition is melted. Load impedance for the upper (primary) coil load circuit is substantially equal to the load impedance for the lower (secondary) coil load circuit. As the metal composition is inductively heated, molten material forms at the bottom of the crucible while solid material is generally added to the upper section of the crucible. FIG. 2(b) illustrates the “warm state” of the batch melting process wherein the lower half of the crucible generally contains molten material (diagrammatically shown as lines) and the upper half of the crucible generally contains solid material. In the warm state the load impedance of the lower coil load circuit is lower than the load impedance of the upper coil load primarily since the equivalent load resistance of the molten material is lower than the equivalent load resistance of the solid material. Finally in FIG. 2(c), which illustrates the “hot state” of the batch melting process, generally all of the material in the crucible is in the molten state, and the load impedances in the upper and lower coil load circuits are equal, but lower in magnitude than the load impedances in the cold state.
FIG. 3(a), FIG. 3(b) and FIG. 3(c) graphically illustrate the division of power supplied from the power supply in the upper (primary section c1i in these figures) and lower (secondary section c2i in these figures) coil sections for the total coil (ci in these figures) shown in FIG. 1 and FIG. 2(a) through FIG. 2(c) as the batch melting process proceeds through the cold, warm and hot stages, respectively. For example: in the cold state (FIG. 3(a) with power supply output at 600 kW and approximately 390 Hertz), approximately 300 kW is supplied to the upper coil section and 300 kW is supplied to the lower coil section; in the warm state (FIG. 3(b) with power supply output at 600 kW and approximately 365 Hertz), approximately 200 kW is supplied to the upper coil section and 400 kW is supplied to the lower coil section; and in the hot state (FIG. 3(c) with power supply output at 600 kW and approximately 370 Hertz), approximately 300 kW is supplied to the upper coil section and 300 kW is supplied to the lower coil section. This example illustrates the general process condition that as the batch melting proceeds from the cold state to the warm state, more power is provided to the lower coil section than to the upper coil section since the lower coil section surrounds an increasing amount of molten material, which has a lower resistance than the solid material, as the process progresses until the height of the molten material is sufficient to magnetically couple with the field generated by the upper coil section. This condition is opposite to the preferred condition, namely that the solid material should receive more power than the molten material to quicken melting of the entire batch of metal. The solid line in FIG. 4 graphically illustrates the typical efficiency of a batch melting process over the time of the process while the dashed line illustrates a typical 82 percent average efficiency for the process.
Similarly when the primary and secondary coil sections surround a susceptor or an electrically conductive material, such as a billet or metal slab, the arrangement in FIG. 1 and FIG. 2(a) through FIG. 2(c), with the susceptor or electrically conductive material replacing crucible 110 containing solid metal composition 112, results in a non-controlled temperature pattern along the length of the material due to the fact that the energy delivery pattern is defined by the coil arrangement and the energy consumption pattern is defined by the processes inside a susceptor, or the heat absorption characteristics of the billet material.
There is a class of materials, such as silicon, that are substantially non-electrically conductive in the “cold” or solid (crystalline) state and electrically conductive in the non-solid (semi-solid, liquid or molten) state. For example the resistivity of crystalline silicon is over 100,000 μohm·cm below its nominal melting temperature of 1,410° C., and typically 75-80 μohm·cm in the molten state. This class of materials is referred to herein as transition materials. Typically a transition material is heated to the molten state to reshape the material or separate impurities from the material. Electric induction power directly heats an electrically conductive material by inducing eddy currents in the material as described above and in FIG. 1 and FIG. 2(a) through FIG. 2(c). If the material is non-electrically conductive, then an indirect induction heating method must be used to heat the material. For example electric induction power can be used to electromagnetically heat a discrete susceptor, with heat from the susceptor being transferred to the transition material by conduction, and then by convection through the transition material.
There are two general approaches to heating and melting a transition material with electric induction power. In the first general approach, “cold” or solid and substantially non-electrically conductive transition material, for example, in the form of pellets, are placed in a non-electrically conductive refractory crucible surrounded by an induction coil. Since flux from the magnetic field generated by the flow of ac current in the coil can not inductively heat the solid transition material, one or more discrete susceptors can either be permanently installed in areas around the non-electrically conductive crucible, or temporally brought close to, or in contact with, the solid transition material in the non-electrically conductive crucible. The magnetic flux will electromagnetically heat (suscept) the discrete susceptors due to their high susceptance, and, in turn, the susceptors will transfer heat by conduction to the solid transition material in the non-electrically conductive crucible. Permanently installed discrete susceptors are disadvantageous in that after the solid transition material begins to melt and becomes electrically conductive, magnetic flux continues to be at least partially coupled with the permanently installed discrete susceptors, which decreases the efficiency of the heating and melting process. Further depending upon where the one or more discrete susceptors are permanently located, relative to other components of the crucible system, dissipation of electromagnetically generated heat in the discrete susceptor can degrade adjacent components of the crucible system. For example an electromagnetically heated discrete susceptor located adjacent to a crucible's interior liner material that prevents contamination of transition material in the crucible with refractory material may overheat and degrade the liner while heat is transferred by conduction from the susceptor to the transition material in the crucible. Temporarily installed discrete susceptors are disadvantageous in that apparatus is required for moving the susceptors. The requirement for susceptors can be eliminated by depositing transition material in the solid state into a refractory crucible that is at least partially filled with molten transition material. The solid material must be quickly dissolved in the molten bath while electromagnetic induction current suscepts to the molten material and provides necessary heat for melting.
In the second general approach, the solid transition material can be placed in a susceptor vessel that is surrounded by an induction coil. The flow of ac current in the induction coil will generate a magnetic field that electromagnetically couples with the susceptor vessel to heat the vessel. The heated susceptor vessel will heat transition material placed in the vessel by conduction regardless of the state of electrical conductivity of the material. The degree to which the magnetic flux from the field will couple with the susceptor vessel and electrically conductive transition material in the susceptor vessel is fundamentally dependent upon the electrical frequency of ac current supplied to the induction coil and the wall thickness of the susceptor vessel. The standard depth of penetration (Δ, in meters) of ac current into a material as a function of frequency is defined by the equation:
                              Δ          =                      503            ·                                          ρ                                  f                  ·                  μ                                                                    ,                            [                  equation          ⁢                                          ⁢                      (            1            )                          ]            
where ρ is the resistivity of the material comprising the susceptor vessel in ohm·meters;
f is the frequency of the ac current supplied to the induction coil in Hertz; and
μ is the magnetic permeability (dimensionless relative value) of the material comprising the susceptor vessel.
If the standard depth of penetration is less than the thickness of the susceptor vessel, then most input electrical energy is used to electromagnetically heat the susceptor vessel, which then transfers heat to the transition material in the vessel by conduction. Conversely if the standard depth of penetration is substantially greater than the thickness of the susceptor vessel, then most input electrical energy is used to inductively heat transition material in the vessel after it transitions to the non-solid state.
Therefore there is the need for selectively inducing heat to a susceptor vessel and a transition material contained in the vessel when the inductive heating and melting process utilizes multiple coil sections.
It is one object of the present invention to provide apparatus for, and method of, batch heating and melting of a transition material with electric induction power in a susceptor vessel surrounded by multiple coil sections without the disadvantages of a refractory crucible in combination with discrete susceptors located either permanently or temporarily around, or in, the refractory crucible while optimizing the transfer of induced power to transition material in the susceptor vessel when the transition material is in the electrically conductive state.
It is another object of the present invention to electromagnetically induce a stirring pattern in the transition material in the susceptor vessel when substantially all transition material is in the electrically conductive molten state to achieve rapid dissolution of any solid transition material that may be added to the molten transition material in the susceptor vessel.